Method for determining temperatures on semiconductor components

ABSTRACT

In order to determine a temperature on a semiconductor component ( 1 ), a scanning light wave ( 7 ) is irradiated onto a measuring point on the semiconductor component, a response light wave ( 8, 8 ′) reflected from the measuring point is recorded, and the temperature of the measuring point is ascertained with the aid of a temperature-dependent property R of the response light wave ( 8, 8 ′).

BACKGROUND INFORMATION

[0001] The present invention relates to a method and a device for determining temperatures on semiconductor components.

[0002] In the field of semiconductor technology, the performance of a component depends strongly, among other things, on its admissible operating temperatures. A very frequent cause of failure is temperatures that are too high during operation, which are able to damage the component severely or completely destroy it. Therefore, both for the user who has to select suitable semiconductor components for a certain application, and for a manufacturer who has to make specifications for the development of such an element, and also for the manufacturer who specifies his product, the knowledge of the temperature of the component, which sets in under certain usage conditions, is of great interest.

[0003] It is state of the art to characterize the temperature of a semiconductor component via the so-called barrier layer temperature T_(j), which can be determined, for example, by measuring the forward voltage of pn junctions of a component. (These are junctions between p-doped and n-doped areas of a semiconductor; they are, for instance, components of rectifier and Zener diodes or are present in the form of an intrinsic body diode of a field-effect transistor). In this context, one makes use of the fact that the voltage U_(flow), which has to be applied to a pn junction in the flow direction in order to let a certain current I flow, is a function of the crystal temperature at the location of the pn junction. Via the functional connection U_(flow)(I,T_(j)), by measuring the forward voltage U_(flow) for a given current flow I, one may conclude what temperature T_(j) is. However, for this one has to know the function U_(flow)(I,T_(j)), which is generally ascertained by a preceding stationary calibration measurement of U_(flow)(I,T_(j)).

[0004] A considerable disadvantage in this known method is, however, that one has to send measuring current I via the component in flow direction. That is, one cannot use this method as long as another operating state of the component prevents this forward current (e.g. during an avalanche breakdown or the like). In addition, in complex integrated circuits the problem may arise that, in general, the transmission states of different semiconductor junctions in the component are not able to be set completely independently of one another. Therefore, the case may arise in which there is an interest in measuring the temperature of a certain pn junction which, however, under normal operating conditions of the component is transparent only transiently. In such a case it is not possible to carry out ahead of time a stationary calibration at the pn junction.

[0005] These peculiarities of the known method lead to the method not being usable, for example, for the investigation of the barrier breakdown (avalanche effect) of a diode. The avalanche state is characterized in that, in the reverse direction, such a high voltage is present that the diode breaks down and a large current flows in the reverse direction (so-called Zener breakdown of a diode). The high fields and currents in general lead to strong heating of the component, the hottest spot being at the pn junction breaking down. In order to determine the temperature prevailing there, using the known forward voltage method, one has to wait, however, until the barrier current has decayed almost completely, in order then to conduct a measuring current through the pn junction in the opposite direction. The result of this is that the known method is first able to be used after a certain time delay after the decay of the Zener breakdown. The temperature present at this point in time, to be sure, no longer corresponds to the temperature spike at the pn junction that appears during the breakdown, but mostly to a clearly lower temperature, because, between the end of the avalanche state and the beginning of the measurement, the heat can distribute itself already from the region of the pn junction to a larger region of the component or even to the thermally interfaced environment of the component. However, in general it is the transient temperature spikes which lead to damage of the component.

[0006] To be sure, the possibility exists of gaining insight concerning the temperature development in the semiconductor component by following the development over time of the forward current after a Zener breakdown, and, by extrapolating this development to points in time before the measurement, of coming to a conclusion on the temperature which could have been prevalent at the time of the breakdown at the interface. However, this method is encumbered with considerable uncertainties. One reason for this is the shortness of the measuring times available and thus the limited accuracy of the temperature measurement which is the more extreme the higher the required temporal resolution; for another reason, there is a fundamental problem in that, by the forward current measurement, only one average value of the temperature is able to be ascertained over the entire surface of the pn junction, but that it is not at all certain that the avalanche current, and thus the temperature distribution in the avalanche state, is uniformly distributed over the junction surface.

SUMMARY OF THE INVENTION

[0007] A method and a device for determining temperatures at semiconductor components is created by the present invention, which make possible a measurement at any time, independent of an operating state prevalent at a semiconductor junction. It is a further advantage of the present invention that it makes possible a temperature measurement having a spatial resolution, using which even irregular current strength distributions may be recorded at a semiconductor junction with the aid of temperatures resulting therefrom. The measurement may be made in time resolution, the resolution being in the millisecond range and below.

[0008] These advantages are achieved by a method for determining a temperature at a semiconductor component having the steps:

[0009] irradiating a scanning light wave on a measuring point on the semiconductor component,

[0010] recording a response light wave reflected from the measuring point,

[0011] recording the temperature of the measuring point with the aid of a temperature-dependent property of the response light wave.

[0012] The advantages are additionally achieved by a device for determining a temperature at a semiconductor component, using a light source for irradiating a scanning light wave onto a measuring point on a semiconductor component, using a light-sensitive element for recording a temperature dependent property of the response light wave and a processing unit for converting a recorded value delivered by the light-sensitive element into a temperature.

[0013] The conversion of an instantaneous value recorded by the light-sensitive element of the temperature-dependent property into a temperature is made preferably by a comparison with values of a reference curve which describes the property as a function of the temperature.

[0014] Such a reference curve is expediently acquired ahead of time under thermally static conditions.

[0015] In order to be able to acquire the reference curve at the same component at which the measurement is also taken later, under identical conditions, the device according to the present invention is expediently furnished with an oven, using which, the temperature of the semiconductor component may be regulated in a stationary manner at a desired value.

[0016] Since temperature-dependent properties of the semiconductor component, such as linear or nonlinear reflection coefficients, index of refraction, absorption coefficient, etc, may vary from place to place, corresponding to the functional structures developed on the semiconductor component, the reference curve is preferably acquired in each case for a single measuring point of the semiconductor component. Expediently, a small surface section or boundary area section of the semiconductor component, which has a homogeneous structure, is selected. At this measuring point of the same semiconductor component or of an element constructed in the same way, the measurements of the heating up of the semiconductor component induced by the operating current flow are then acquired.

[0017] One may define several such measuring points on a semiconductor component, and reference curves may be acquired for such measuring points which belong to different pn junctions or even to regions, separated in space, of a similar pn junction.

[0018] On an unpackaged component or one exposed from its packaging, the temperature distribution at the surface may thus be, so to speak, “mapped”.

[0019] If only a few particular temperature measuring points are needed, it may be sufficient to expose these by making a small bore hole of a few square millimeters in cross section in the packaging.

[0020] Preferably, a sufficiently intense monochromatic light source, in particular a laser beam, is used as the scanning light wave. Sufficiently intense means that the light has to be intense enough so that the response signal may be interpreted. An excessively high intensity would, under certain circumstances, heat up the component and invalidate the measurement, in case the radiation were correspondingly absorbed.

[0021] The response light source can then be recorded at the frequency of the scanning light wave; at sufficient intensity of the scanning light wave, it is also possible to record the harmonics when there are multiples of the frequency of the scanning light wave. When two monochromatic scanning light waves are used, the response light source can also be recorded by using the sum or the difference of the frequencies of the two scanning light waves. Such a procedure makes possible an increase in intensity of the response light wave by tuning at least one scanning light wave to a resonance of the material of the component.

[0022] The response light wave can be recorded in the Fresnel reflection direction of the scanning light wave, but it is also conceivable to evaluate a response light source which is scattered nondirectionally in other spatial directions than the Fresnel reflection direction.

[0023] In a preferred way, a wavelength is selected for the scanning light wave to which the substrate material of the semiconductor component is transparent. In this manner, it is avoided, on the one hand, that, by the absorption of the scanning light wave in the semiconductor component, the latter is heated and that the measuring result is thereby invalidated, and on the other hand, that, by the immission of the scanning light wave, free charge carriers are induced in the semiconductor material, which might affect the switching performance of the semiconductor component. Semiconductor materials generally have transparent ranges in the near infrared at wavelengths above 1 μm.

[0024] Further features and advantages of the present invention are seen in the following detailed description of exemplary embodiments, with reference to the enclosed figures. The figures show:

[0025]FIG. 1 a schematic view of the measuring device according to the present invention.

[0026]FIG. 2 an example of the development over time of the reflectivity of the surface of a semiconductor element and the development of the temperature ascertained therefrom.

[0027]FIG. 3 a measuring method which is suitable for use as a light source using a pulsed laser.

[0028]FIG. 4 the spatial pattern of the scanning and response light wave in a case in which the second harmonic of the scanning light wave is recorded as the response light wave.

[0029]FIG. 5 the spatial pattern of the scanning light wave and the response light wave when the classically reflected or the transmitted scanning light wave is recorded as the response light wave.

[0030]FIG. 1, in a very schematic way, shows a device according to the present invention for determining temperatures at a semiconductor component 1. In the device shown here, semiconductor component 1 is situated on an arrangement of relocating stages 2, 3 positioned displaceably in two perpendicular directions to the exposed surface of component 1 (with reference to the figure in the horizontal direction and perpendicularly to the plane of the drawing). Component 1 is surrounded by a thermostatically regulated oven 4 which has an entrance window and an exit window 5, 6 for a scanning light wave 7 and a response light wave 8. As light source for generating the scanning light wave a laser 9 is used, such as a solid-state laser in which rare earth ions are used as laser-active species. Depending on the kind of ions used and their carrier crystal, such lasers may have wavelengths in the range of ca 1.0-1.5 μm.

[0031] Scanning light wave 7 generated by laser 9 is guided to a measuring point on the surface of semiconductor component 1; response light wave 8 starting from there runs through a filter 10 and is picked up by a light-sensitive element 11, such as a PIN photodiode, a pyroelectric detector or the like. Filter 10 is used for screening light-sensitive element 11 from the surrounding light. Depending on the intensity of the scanning light wave expected, filter 10 may be a simple colored glass filter or an interference filter, but under certain circumstances, which are discussed in greater detail below, the use of a grating monochromator may be required.

[0032] A beam portion split from scanning light wave 7 by a beam splitter 12 is guided to a second light-sensitive element 13.

[0033] An evaluation and processing unit 14 is connected to both light-sensitive elements 11, 13 for calculating the ratio of the intensities of the waves picked up by the two light-sensitive elements, and thus to ascertain the reflection coefficient R of the measuring point. Furthermore, a temperature sensor 15 is connected to an evaluation and processing unit 14, which is used for recording the temperature on the inside of oven 4.

[0034] In a first phase of the method according to the present invention, a reference curve is generated which shows the dependence of the reflection coefficient of the surface of semiconductor element 1 on its temperature. For this purpose, oven 4 is slowly heated together with semiconductor component 1 present in it, and during the heating, reflection coefficient R of the surface is ascertained by forming the relationship of the intensity signals delivered by the light-sensitive elements 11, 13, and is stored in a memory of evaluation/processor unit 14, as a function of temperature T prevailing in each case in oven 4 at the point in time of the ascertainment.

[0035] If the surface of semiconductor component 1 is structured and has a different reflection coefficient from location to location, such a reference curve is acquired for a plurality of measuring points at which temperature measurements are to be carried out later under operating conditions of the semiconductor component.

[0036] As the first example for the application of the reference curve for ascertaining a temperature of semiconductor component 1 and the operating conditions, the case is considered in which these operating conditions are stationary. In this case it is sufficient to irradiate scanning light waves 7 onto semiconductor component 1 under the exact same geometrical conditions as during the acquisition of the reference curve, and to pick up response light wave 8 reflected from its surface in order to compare its temperature-dependent intensity with the intensity picked up by light-sensitive element 13. The comparison yields a reflection coefficient R which, with the aid of the reference curve, can be clearly associated with a temperature of semiconductor component 1.

[0037] It is true that with the use of this method only slow temperature changes of semiconductor component 1 are able to be recorded. However, to a developer and user of semiconductors the knowledge is meaningful of temperatures which may occur transiently during the course of a switching operation, that is, in the non-stationary operation of the component. One principle of the recording of such transient temperature patterns is illustrated in FIG. 2. In order to measure a temperature pattern on semiconductor 1, a switching operation which leads to heating up is repeated cyclically. FIG. 2 shows, in a first partial diagram, as curve R(t) the pattern of reflectivity R as a function of time t, over two switching cycles. Actually, the reflectivity is not measurable with the accuracy of the curves shown, but is strongly noise-infested, so that expediently, with the aid of a lock-in amplifier, which may be a part of evaluation and processing unit 14, and which is coupled to the switching cycle of component 1, an average curve of reflectivity R is ascertained during the course of one cycle, which is subsequently converted with the aid of reference curve R(T) into a time-dependent curve of temperature T(t).

[0038] This procedure is suitable for use for the scanning wave in conjunction with a continuous light source.

[0039]FIG. 3 illustrates a measuring method which is suitable for use as a light source using a pulsed laser. In FIG. 3 three curves are shown, respectively superimposed. A first curve 20 shows reflectivity R of the surface of semiconductor component 1 as a function of time t. The curve repeats in each operating cycle of the semiconductor component for a duration of t₁. A second curve 21 shows the time-dependent intensity of the pulsed laser, and its period t₁+ε differs by a very small, non-vanishing value ε from period t₁ of the operating cycle. In the figure this difference ε is shown in an exaggerated fashion, in order to make clear the way in which, during the course of time t, the position of the laser pulses of curve 21 shifts with respect to the operating cycle of component 1. With each laser pulse, curve 20—in respectively different cycles—is scanned using a different phase relation, and a scanning value for the reflection coefficient is obtained which in each case corresponds to another phase relation of cyclical curve 20. The result of the scanning values thus obtained yields a curve 22, whose pattern, on a stretched time scale, corresponds to the pattern of reflectivity curve 20 in each individual cycle. By selection of the value ε in the order of magnitude of the laser pulse duration, the factor of the stretching can be determined; one may achieve a temporal resolution of the reflectivity measurement corresponding to the duration of the laser pulse.

[0040] In the description so far it was assumed that the response light wave, which is picked up and evaluated for ascertaining of the temperature, comes about from the scanning light wave by reflection at the surface of the semiconductor component according to the classical laws of Fresnel optics. This does not necessarily have to be the case. Thus, for example, it is conceivable that one might not pick up the reflected ray as the response light wave, but rather light which is diffusely reflected at the surface of component 1.

[0041] It may also prove expedient, instead of using, as usual, linear reflected light, to take advantage of nonlinear optical phenomena at the surface, or even at another boundary area of the semiconductor component. The simplest possibility for this is frequency doubling at the surface. Optical frequency doubling or, more generally, sum and difference frequency mixing are nonlinear optical procedures which are able to occur in media that are not inversion-symmetrical. In the case of the usual semiconductor materials, a break in the inversion symmetry exists only at boundary surfaces. That is why sum and difference frequency mixing occurs exclusively at boundary surfaces. But it is particularly the boundary surfaces, e.g. between differently doped zones in the semiconductor component or between the semiconductor substrate and a metallization, whose temperatures are important to know, in order to be able to optimize the power-handling capacity of semiconductor components, and also to be able to optimize models which permit simulating temperature distributions in a semiconductor component computationally.

[0042] An advantageous variant of beam control at semiconductor component 1 while using the second harmonic as response light wave 8 is shown in FIG. 4. Here, scanning light wave 7, at two equal and opposite angles to the normal to the surface, hits a measuring point at the surface of semiconductor component 1. Because of the nonlinear interaction of the light coming in from two different directions with the semiconductor surface, frequency-doubled light is created which is radiated, concentrated in part, in the direction of the normal to the surface. This response light wave is practically free from background having the frequency of the scanning light wave. Because of this, although the intensity of the frequency-doubled response light wave is smaller by several orders of magnitude than the scanning light wave, the response light wave, possibly with the use of further filters or a monochromator, can be detected, separated from the background and with the aid of a photomultiplier as light-sensitive element 11.

[0043] According to another further refinement, a polarizer may be provided in the optical path of the scanning light wave, independently of whether it is a light wave at the frequency of the scanning light wave or a harmonic thereof, in order to record the intensity of the response light wave as a function of the polarization. If laser 9 supplies polarized light, this polarizer may be oriented particularly orthogonally to the direction of polarization, in order only to record a depolarized component in the light reflected by component 1 as response light wave, and thus to suppress a major portion of the intensity of the reflected light. If an non-polarized light source is used, a second polarizer for polarizing the scanning light wave can of course be inserted between the light source and the surface of the semiconductor component.

[0044] In the present description we have assumed that response light wave 8 is emitted at the surface of semiconductor component 1 into the half-space from which the scanning light wave hits the surface. As shown in FIG. 5, however, there is also the possibility of picking up the portion of scanning light wave 7 transmitted by semiconductor component 1 as response light wave 8′, using a light-sensitive element 11′, and thus to record the likewise temperature-dependent absorption coefficient of semiconductor material 1. While for reflection measurements one would prefer to select a wavelength of scanning light wave 7 in a range of complete transparency of the semiconductor substrate, for transmission measurements a wavelength is advisable at the edge of the transparency range, so that even minor temperature-dependent changes in the absorption coefficient lead to a measurable intensity change of transmitted response light wave 8′. By using beam splitter 12 and recording the split-off ray in light-sensitive element 13, in evaluation or calculating unit 14, even in the example as in FIG. 5, absolute coefficients may be determined, both the absolute reflection coefficient and transmission coefficients, from the relationship of the signals of the detectors or light-sensitive elements 11′ and 13. If one measures reflection and transmission at the same time, this is possible in the case of certain sample geometries and requires a further detector element, that is, altogether three detector elements 11, 11′, 13, and then the absorption coefficient of semiconductor material 1 and possibly the refractive index can also be ascertained from the signals received. Both values may be used for the temperature determination, as was described in connection with the example as in FIG. 1. 

What is claimed is:
 1. A method for determining a temperature on a semiconductor component (1), using the steps: irradiating a scanning light wave (7) on a measuring point on the semiconductor component, recording a response light wave (8, 8′) reflected from the measuring point, recording the temperature of the measuring point with the aid of a temperature-dependent property (R) of the response light wave (8, 8′).
 2. The method as recited in claim 1, wherein the temperature is recorded by comparing an instantaneous value (R(t)) of the temperature-dependent property with values of a reference curve (R(T)).
 3. The method as recited in claim 2, wherein the reference curve (R(T)) is acquired ahead of time under thermally static conditions.
 4. The method as recited in claim 2 or 3, wherein reference curve (R(T)) is in each case acquired specifically for a single measuring point of the semiconductor component (1).
 5. The method as recited in one of the preceding claims, wherein the scanning light wave (7) is monochromatic and the response light wave (8, 8′) is recorded at the frequency of the scanning light wave (7).
 6. The method as recited in one of claims 1 through 4, wherein the scanning light wave (7) is monochromatic and the response light wave (8) is recorded at a multiple of the frequency of the scanning light wave (7).
 7. The method as recited in one of claims 1 through 4, wherein two monochromatic scanning light waves (7) are used and the response light wave (8) is also able to be recorded by using the sum or the difference of the frequencies of the two scanning light waves (7).
 8. The method as recited in one of the preceding claims, wherein the temperature-dependent property is the intensity or the polarization of the response light wave (8).
 9. The method as recited in one of the preceding claims, wherein the response light wave (8) is recorded in the Fresnel reflection direction of the response light wave.
 10. The method as recited in one of claims 1 through 8, wherein the response light wave (8) is obtained by nondirectional scattering at the surface of the semiconductor component (1).
 11. The method as recited in one of the preceding claims, wherein a wavelength is selected for the scanning light wave (7) at which the substrate material of the semiconductor component (1) is transparent.
 12. A device for determining a temperature on a semiconductor component, having a light source (9) for irradiating a scanning light wave (7) onto a measuring point on the semiconductor component (1), a light-sensitive element (11) for recording a temperature-dependent property of the response light wave (8) reflected from the measuring point, characterized by a processing unit (14) for converting a recorded value supplied by the light-sensitive element (11) into a temperature.
 13. The device as recited in claim 12, wherein the processing unit (14) has a memory for at least one reference curve (R(T)) of the temperature-dependent property as a function of the temperature.
 14. The device as recited in one of claims 12 through 13, characterized by an oven (4) for regulating the temperature of the semiconductor component (1).
 15. The device as recited in one of claims 12 through 14, characterized by at least one actuator (2, 3) for shifting the semiconductor component (1) relative to the light source (9) and the light-sensitive element (11).
 16. The device as recited in one of claims 12 through 15, wherein the light source (9) is a laser.
 17. The device as recited in one of claims 12 through 16, wherein the light-sensitive element (11) is a photomultiplier.
 18. The device as recited in one of claims 12 through 17, wherein a polarizer is situated between the semiconductor component (1) and the light-sensitive element (11). 